Mixing bonding adhesive at die bonder before dispense

ABSTRACT

A method for die bonding includes positioning a dispenser in a die bonding apparatus, wherein the dispenser includes a reservoir having bonding adhesive therein including particles and a liquid carrier. The dispenser is moved to provide mechanical agitation to the dispenser for mixing the bonding adhesive into a homogeneous mixture of particles and the liquid carrier, wherein the bonding adhesive is not dispensed during moving. After the moving, the bonding adhesive is dispensed onto a bonding location on the workpiece without removing the dispenser from the die attach apparatus. An integrated circuit (IC) die is attached onto the bonding adhesive over the bonding location. The method can also include determining an amount of time elapsed after the last mixing of the bonding adhesive or the positioning of the dispenser in the die bonding apparatus, and automatically initiating movement for mixing only if the elapsed time exceeds a predetermined time.

FIELD

Disclosed embodiments relate to semiconductor package production, morespecifically to die bonding.

BACKGROUND

As known in the art, an integrated circuit (IC) die is attached to aworkpiece such as a package substrate, lead frame or a carrier duringpackaging of IC devices. The package substrate or carrier is generallymade of an electrically insulating (i.e., dielectric) material, forexample, ceramic or plastic. The process by which the IC die is attachedto the workpiece is known as die bonding. There are different techniquesof die bonding including, for example, eutectic bonding, preformbonding, and adhesive bonding.

Adhesive bonding includes using an adhesive paste, also known as a dieattach or die bonding adhesive, to attach the IC die to a workpiece. Thebonding adhesive can be dispensed on the workpiece and the IC die placedover it. The adhesive may include an epoxy monomer or another suitableliquid carrier, and generally includes a suspension of filler particlessuch as metal filler particles in the carrier. As known in the chemicalarts, a suspension is a heterogeneous fluid containing solid particlesthat are sufficiently large (generally at least 1 μm in size) forsedimentation. The internal phase (solid) is dispersed throughout theexternal phase (liquid) through mechanical agitation for mixing beforeloading into the die bonder. Unlike colloids, suspensions have thetendency to settle over a period of time which results in separation ofthe solid particles and the liquid.

The metal particles can be silver particles, although other metals orelectrically conductive materials may be used, e.g., gold, nickel,copper, or graphite, etc. It is generally desirable to load the metalparticle content as high as possible in order to obtain the highestpossible thermal and electrical conductivity. The particles aregenerally denser as compared to the liquid carrier. Following dieattach, a heated curing step is generally used that polymerizes theliquid carrier. Curing the carrier provides adhesion to make a bond toboth the IC and the workpiece with the required mechanical strength, andthe metal particles provide electrical or/and thermal conductivity. Theadhesive can thus be electrically conductive or non-conductive, andthermally conductive or non-conductive.

In a known technique of adhesive bonding for an adhesive comprisingmetal filler particles in a carrier, the uncured adhesive is mixed wellwhile in the dispenser which includes a reservoir for the bondingadhesive by mechanical agitation provided by a stand-alone mixer, suchas a combined centrifuge mixer, to resuspend the filler to provide ahomogeneous, uniform suspension. After mixing, the dispenser is thensecured to the die bonder apparatus. While in the die bonder apparatusthe suspension is then injected from the dispenser onto the workpiece,which is followed by attachment of the IC die. The dispenser may includea syringe having an end aperture.

During normal production, the metal or other particles tend to remain insuspension because dispensing includes some X-Y motion which helpsprevent precipitation so that separation of the denser filler particlesthat can otherwise cause delamination of the die attach adhesive fromthe IC device is typically not a significant problem. As a method ofprocess control, X-ray inspection can be used after die bonding to checkfor delamination.

SUMMARY

Disclosed embodiments recognize that for particle filled carrier-baseddie bonding adhesives, the relatively dense particles (e.g., metalparticles) may separate and settle down into the bottom of the dispenser(e.g., syringe) due to gravity over a period of time, rendering theadhesive non-homogeneous, which can cause delamination between theintegrated circuit (IC) die and the workpiece in the semiconductor ICpackage. For example, if an abnormality (e.g., index jam due toleadframe warpage, a lead frame load or unload issue due to a mechanicalproblem, alignment failure on dispensing/bonding/wafer area, die pick upfailure, etc.) happens on the die bonder, the abnormality can cause along idle time without any dispensing, such as one hour or more. Therate of separation increases as the particle loading increases, whichcan aggravate the delamination problem. Although delamination may bedetected using conventional X-ray inspection after die bonding, one ofthe drawbacks of X-ray inspection is that the inspection does notaddress the root cause, i.e., the separation of the particles in thecarrier (e.g., epoxy).

Disclosed embodiments add mechanical agitation while the die bondingadhesive is in the dispenser and while the dispenser is within the dieattach apparatus before dispensing so that the die bonding adhesive asdispensed on the bonding sites stays within a specified homogeneity. Themechanical agitation thus reverses separation of the denser particles(e.g., metal particles) from the carrier to address the root cause ofthe delamination problem by keeping the bonding adhesive well mixedbefore dispensing.

An example method for die bonding comprises positioning a dispenser in adie bonding apparatus, wherein the dispenser includes a reservoir havingbonding adhesive therein comprising particles and a liquid carrier. Thedispenser is moved to provide mechanical agitation to the dispenser formixing the bonding adhesive into a homogeneous mixture of particles andliquid carrier, wherein the bonding adhesive is not dispensed duringmoving. After the moving, the bonding adhesive is dispensed onto abonding location on the workpiece without removing the dispenser fromthe die attach apparatus. An IC die or die stack is attached onto thebonding adhesive over the bonding location. The method can also includedetermining an amount of time elapsed after the last mixing of thebonding adhesive or the positioning of the dispenser in the die bondingapparatus, and automatically initiating moving of the dispenser toprovide mixing only if the elapsed time exceeds a predetermined time.

An example die bonding apparatus comprises a moveable dispenser assemblyincluding at least one dispenser having a reservoir for holding abonding adhesive comprising particles and a fluid carrier. The dispenseris for dispensing a volume of bonding adhesive onto a surface of aworkpiece in the bonding location. Movement of the moveable dispenserassembly provides mechanical agitation to the dispenser for mixing thebonding adhesive into a homogeneous mixture of the particles and liquidcarrier. An opening at an end of the dispenser is for dispensing thebonding adhesive to the surface of the workpiece for attaching an IC dieor die stack. At least one controller is provided for sending a controlsignal that triggers movement of the moveable dispenser assembly formixing the bonding adhesive before dispensing the volume of bondingadhesive onto the surface of the workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart that shows steps in an example method for diebonding including moving a dispenser having bonding adhesive thereinbefore dispensing, according to an example embodiment.

FIG. 2 illustrates a portion of an example die bonding apparatus thatcomprises a moveable dispenser assembly for moving a dispenser havingbonding adhesive therein before dispensing, according to an exampleembodiment.

DETAILED DESCRIPTION

Example embodiments are described with reference to the drawings,wherein like reference numerals are used to designate similar orequivalent elements. Illustrated ordering of acts or events should notbe considered as limiting, as some acts or events may occur in differentorder and/or concurrently with other acts or events. Furthermore, someillustrated acts or events may not be required to implement amethodology in accordance with this disclosure.

FIG. 1 is a flow chart that shows steps in an example method 100 for diebonding including moving a dispenser having bonding adhesive thereinbefore dispensing, according to an example embodiment. Step 101comprises positioning a dispenser in a die bonding apparatus, whereinthe dispenser includes a reservoir having bonding adhesive thereincomprising particles and a liquid carrier. The dispenser is generallysecured to a moveable dispenser assembly, such as an X-Y table. Thedispenser can comprise a syringe including an opening on an end thatdispenses the bonding adhesive upon application of an applied dispensingpressure.

The particles can comprise electrically conductive, semiconductor ordielectric particles, or mixtures thereof. The bonding adhesive can alsobe referred to as die attach adhesive. The bonding adhesive may be apaste including an epoxy as a liquid carrier (e.g., epoxy bonding). Inone embodiment, the bonding adhesive comprises a paste including epoxyand the particles comprise silver particles suspended in the paste. Theadhesive may alternatively include another suitable liquid carrier, forexample, silicone, polyurethane, or cyanoacrylate. The particles aregenerally sufficiently large (generally at least 1 μm in size) forsedimentation, such as silver particles averaging several μms in size ina solvent. The carrier provides adhesion and cohesion between the dieand the substrate. Metal particles can provide electrical and/or thermalconductivity. The adhesive can be an electrically conductivesilica-filler or an insulating silica-filler. Silver particles can beused when both good electrical connection and good thermal performanceare desired since they provide both good electrical and thermalconductivity.

Step 102 comprises moving the dispenser to provide mechanical agitationto the dispenser for mixing the bonding adhesive into a homogeneousmixture of particles and liquid carrier, such as to achieve a minimumspecified homogeneity level, wherein the bonding adhesive is notdispensed during moving. The moving can be a shaking or oscillation, oran X-Y movement pattern analogous to the movement used to pattern thebonding adhesive during dispense, or other movement pattern that enablesobtaining a minimum specified homogeneity level. The moving can beperformed continuously after the dispenser is positioned in the diebonding apparatus, before dispensing is initiated. In one embodiment themixing provided by step 102 is sufficient to allow the conventionalmixing in a stand along centrifugal mixer or other mixer before loadingthe dispenser in the die bonding machine to be eliminated.

In step 103, after the moving, the bonding adhesive is dispensed onto abonding location on a workpiece without removing the dispenser from thedie attach apparatus. The bonding adhesive is dispensed in a pattern andvolume appropriate for the die size. The workpiece, is moved from aninput position to the bonding location sometimes referred to as thedispense position. The workpiece can comprise a package substrate, suchas a ceramic or organic substrate, lead frame, or a carrier.

Step 104 comprises attaching an IC die or die stack onto the bondingadhesive over the bonding location. For example, a pattern recognitionsystem can locate a good die on a sawn wafer. A vacuum pick-up toolmounted on a bond head can grab the aligned good die from the wafer andplace it on a programmed and pre-dispensed bonding location on theworkpiece.

Optional step 105 comprises comparing a parameter to a predeterminedlimit or range, and automatically initiating moving the dispenser onlyif the parameter is outside the predetermined limit or range. In oneembodiment the parameter comprises an amount of elapsed time after thelast moving of the dispenser or the positioning of the dispenser in thedie bonding apparatus, wherein moving is performed only if the elapsedtime exceeds a predetermined time. In one particular embodiment thepredetermined time is at least one hour. In another embodiment, theparameter comprises a measure of homogeneity (e.g., a specifichomogeneity) of the bonding adhesive, wherein the moving is performedonly if the measure of homogeneity is below a minimum specifiedhomogeneity level.

FIG. 2 illustrates a portion of an example die bonding apparatus 200that comprises a moveable dispenser assembly for moving a dispenserhaving bonding adhesive therein before dispensing, according to anexample embodiment. The moveable dispenser assembly 205 includes a maindispenser 210A positioned in a main dispenser holder 212A and anoptional auxiliary dispenser 210B positioned in an auxiliary dispenserholder 212B. The dispensing holders 212A, 212B are secured to a moveableX-Y table 225. The dispensers 210A, 210B include a syringe having areservoir 213 and an opening (aperture) 214 on an end.

Although two dispensers 210A, 210B are shown in FIG. 2, only onedispenser is generally needed to practice the embodiments describedherein. Multiple dispensers can increase the UPH (Units PerHour)/productivity of the die bonding apparatus 200 when the bottleneckof the die bonding apparatus is at dispensing, as opposed to diemounting. For example, large die size can result in the bottleneck atdispensing, since it will take more dispensing time to complete anadhesive pattern. Multiple dispensers can also provide differentadhesive compositions (e.g., different epoxy types) on a single die pad.The main dispenser 210A and the auxiliary dispenser 210B include abonding adhesive that can be injected onto a workpiece to attach an ICdie to the workpiece, such as a ceramic or organic substrate orleadframe.

As described above, the bonding adhesive can become a non-homogeneous,non-uniform composition over time due to separation of the suspensioninto denser particles from the less dense liquid carrier. The moveabledispenser assembly 205 is designed to be moveable for moving andpositioning the main dispenser 210A and the auxiliary dispenser 210B fordispensing the bonding adhesive to attach the IC die and workpiece undercontrol of a controller 215 that is communicably coupled to componentsin the die bonding apparatus 200 including the moveable dispenserassembly 205. For example, the moveable dispenser assembly 205 can becontrolled to provide movement including oscillation or rotation of themain dispenser 210A and the auxiliary dispenser 210B. The oscillation,rotation or other movement can be initiated and controlled by thecontroller 215. The oscillation or rotation of the main dispenser 210Aand the auxiliary dispenser 210B generates a centrifugal force and acentripetal force on the main dispenser 210A and the auxiliary dispenser210B so as to shake the dispensers 210A and 210B. The movement of thedispensers causes mixing of the adhesive in the main dispenser 210A andthe auxiliary dispenser 210B to ensure a specified homogeneity level forthe bonding adhesive before dispensing for attaching the IC die and theworkpiece. Software or firmware of the controller 215 can be configuredto provide for control of movements such as oscillations, e.g.,automatic periodic oscillations, to keep the bonding adhesive wellmixed.

According to one embodiment, the main dispenser 210A and the auxiliarydispenser 210B are filled with adhesive including particles. The maindispenser 210A and the auxiliary dispenser 210B are then positioned inthe main dispenser holder 212A and the auxiliary dispenser holder 212B,respectively, and movement such as oscillation is applied to themoveable dispenser assembly 205 to pre-mix the adhesive in the main andauxiliary dispensers, 210A, 210B, before dispensing or injecting thebonding adhesive. According to one embodiment, the oscillation isapplied periodically, before dispensing, after the lapse of apredetermined elapsed time interval after the dispenser has beenpositioned in the moveable dispenser assembly. The timer 235 can measurethe time parameter.

The time interval is a function of keeping the adhesive well mixed so asto not have settling of the metal or other particles prior to dispensingthat might cause the delamination. The predetermined time interval maybe a function of the particular adhesive and/or particles, andconcentration of metal particles employed. The predetermined time fordifferent bonding adhesives can be determined through simulation orempirically and stored in a memory 216 associated with the controller215. The controller 215 is coupled to the moveable dispenser assembly205 and software running in the controller can control the oscillationof the moveable dispenser assembly based on the predetermined time forthe different bonding adhesives stored in the memory 216.

For example, the time period might be one, two to three hours forcertain adhesives, and vary from this range for others. The oscillatingis done for a sufficient period of time to ensure the adhesive is wellmixed to achieve a minimum level of homogeneity. For example, the movingsuch as oscillating may be done for about a minute for certainadhesives, and vary from a minute for others. Referring to the timeperiod after the dispenser has been positioned in the moveable dispenserassembly and before the dispensing, this time period may also bereferred to as an idle time for the production system. According toanother embodiment, the oscillation is performed continuously after thedispenser has been operably placed in the dispenser holder, beforedispensing is initiated.

Although not shown in FIG. 2, die bonding apparatus 200 can include anoptical system for monitoring the physical stability of the suspensionreal-time while the bonding adhesive is in the dispensers 210A, 210B,and can use this information to know when to initiate moving thedispenser to accomplish mixing, and stop mixing some time afterinitialization. Multiple light scattering coupled with vertical scanningis a method that can be used to monitor the dispersion state of abonding adhesive. When light is sent through the bonding adhesive, it isbackscattered by the particles. The backscattering intensity is directlyproportional to the size and volume fraction of the dispersed phase.Therefore, local changes in concentration (sedimentation) and globalchanges in size (flocculation, aggregation) can be detected, monitoredand quantified.

Those skilled in the art to which this disclosure relates willappreciate that many other embodiments and variations of embodiments arepossible within the scope of the claimed invention, and furtheradditions, deletions, substitutions and modifications may be made to thedescribed embodiments without departing from the scope of thisdisclosure.

What is claimed is:
 1. A method for die bonding, comprising: positioninga dispenser in a die bonding apparatus, wherein said dispenser includesa reservoir having bonding adhesive therein comprising particles and aliquid carrier; comparing a parameter to a predetermined limit or range;moving said dispenser to provide mechanical agitation to said dispenserfor mixing said bonding adhesive into a homogeneous mixture of saidparticles and said liquid carrier if said parameter is outside saidpredetermined limit or range, wherein said bonding adhesive is notdispensed during said moving; after said moving, dispensing said bondingadhesive onto a bonding location on a workpiece without removing saiddispenser from said die attach apparatus, and attaching an integratedcircuit (IC) die onto said bonding adhesive over said bonding location.2. The method of claim 1, wherein said particles comprise electricallyconductive particles.
 3. The method of claim 2, wherein the bondingadhesive comprises a paste including epoxy and said electricallyconductive particles comprise metal particles suspended in said paste.4. The method of claim 1, wherein said dispenser includes a syringeincluding an opening at an end that dispenses said bonding adhesive uponapplication of an applied dispensing pressure.
 5. The method of claim 1,wherein said parameter comprises an amount of elapsed time after saiddispenser has been last moved or when said dispenser was positioned insaid die bonding apparatus, and wherein said moving is performed only ifsaid elapsed time exceeds a predetermined time.
 6. The method of claim5, wherein the predetermined time is at least one hour.
 7. The method ofclaim 1, wherein said parameter comprises a measure of homogeneity ofsaid bonding adhesive, and wherein said moving is performed only if saidmeasure of homogeneity is below a minimum specified homogeneity level.8. The method of claim 1, wherein said moving said dispenser comprisesoscillating.
 9. The method of claim 1, wherein said moving is performedcontinuously after said dispenser is positioned in said die bondingapparatus, before said dispensing is initiated.